I think you’ve stumbled into the local version of “Is Photography Art?” TLDR / IANAL-TINLA: legally yes in most countries, ethically yes by general consensus – photos taken of what already exists, whether that’s an aspect of nature or manmade works, including artworks, can themselves be art because the photographer makes artistic choices on composition, lighting, etc that can make a photo more than just a visual copy of what already exists. By the same token, your renders of somebody else’s models are your art, not theirs (which doesn’t mean anybody should claim credit for modeling work they didn’t do, but that’s a different issue). Bringing this back to the Weekend Challenge, the challenge is not about modeling! It’s “create a cool image” etc. Although the rules mention models a lot, they were written about twenty years ago, well before Blender’s Grease Pencil (for example) was a decent artistic tool in its own right.
Going back through the Weekend Challenges anyone can see multiple examples of in-ambit high-workload models in average renders getting far fewer votes than simpler stuff artistically shown to advantage in a “cool image”.
But I’ve been participating for over a year, and in that time I’ve seen several discussions of which rules are currently considered non-obsolete, what the rules and pure/open catagories really mean, if it’s time for a rules update, and what the upcoming rules should be. I think it’s good for people, especially confused newbies, to bring up these things because it keeps the conversation live and makes it clear these are still issues.
Anyway, here’s a list of links on the subject/s that might be helpful:
Purity of objects generated by Blender’s included Addons
Human figure – how much has to be done during the challenge for purity?
“If I use textures from a 3rd party on models I made myself, should I mark it as open or pure?”
“ . . . the primary point of the Weekend Challenge . . . ”
Outside textures’ impact on purity